Ask any factory accountant in Ahmedabad or Coimbatore how they handle a GSTR-1 error and, until recently, you would get the same answer: “adjust it in 3B.” An invoice missed, a value typed wrong, a credit note booked late — whatever the sales return said, the summary return was where the real number went in, and the two were reconciled at year-end if anybody got around to it.

That workflow is now obsolete, and a lot of factories have not noticed. The gap between how GST returns are actually filed on the shop floor and how the portal now enforces them is where 2026’s notices are coming from.

What changed: GSTR-1 became the master return

Three separate changes, stacked over about eighteen months, converted GSTR-1 from a disclosure document into the thing that determines what you owe:

Put together: the sales return is now the source of truth, the correction window is measured in days rather than years, and the door eventually locks permanently. The five mistakes below were all survivable in 2023. None of them are now.

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GSTR-1 deadlines and late fees, 2026

Monthly filers (aggregate turnover above ₹5 crore) file by the 11th of the following month. QRMP filers file quarterly by the 13th of the month after the quarter, with the optional Invoice Furnishing Facility by the 13th for the first two months. Late fee is ₹50 per day (₹20 for a nil return), capped at ₹2,000 up to ₹1.5 crore turnover, ₹5,000 between ₹1.5 and ₹5 crore, and ₹10,000 above ₹5 crore.

The five mistakes

Mistake 1 · Still treating GSTR-3B as the correction layer

The invoice was uploaded at the wrong taxable value, or a sales return never made it into the credit-note table. The accountant plans to “net it off in 3B” on the 20th — and discovers the field is greyed out.

At that point there are only two options, and both are worse than getting it right the first time. Either file GSTR-1A before GSTR-3B (one shot, and only if you catch it in time), or pay tax on the wrong number this month and carry the correction into a later period’s amendment table.

Cost: tax paid on a figure you know is wrong, plus interest exposure if the correction runs the other way. Fix: move the reconciliation earlier. The comparison against your books belongs before GSTR-1 is filed, not between GSTR-1 and GSTR-3B.

Mistake 2 · A wrong recipient GSTIN — the one thing GSTR-1A cannot fix

Two customers in the same group, two GSTINs, and the invoice goes out against the wrong one. Or a customer changed state registration and nobody updated the master. Every other field — value, tax, HSN, invoice number — can be amended in GSTR-1A. The recipient’s GSTIN cannot.

Correcting it means an amendment in a later GSTR-1, which means your customer’s GSTR-2B does not show the invoice this month. In practice, that is the phone call where a buyer withholds payment until their credit appears.

Cost: a customer’s ITC delayed by at least a month, and usually a held payment. Fix: validate customer GSTINs at master-data level, not at invoice level. If a GSTIN has never been verified against the portal, no invoice should be raisable against it.

Mistake 3 · Failing the Table 12 HSN summary

Phase 3 of the HSN reporting rollout turned Table 12 from a formality into a gate. Manual typing of HSN codes is disabled — codes come from a dropdown only. Table 12 is split into separate B2B and B2C tabs. Table 13, the document summary, is mandatory. And the HSN-wise values have to agree with the invoice tables; a rate-wise variance beyond a small tolerance stops the return from being submitted.

The digit requirement catches growing factories in particular: four digits if last year’s aggregate turnover was up to ₹5 crore, six digits above it. Cross the threshold and every item master with a four-digit HSN is suddenly non-compliant. The September 2025 rate rationalisation — 12% and 28% withdrawn, 5% and 18% as the standard pair, 40% for demerit goods — added a second trap: item masters still carrying a retired rate throw a mismatch the moment they meet Table 12.

Cost: the return will not submit at all, usually discovered on the 11th. Fix: HSN and GST rate are item-master fields with validation, not free text on the invoice. Audit the masters once when turnover crosses ₹5 crore, and once after any rate notification.

Mistake 4 · B2C inter-state invoices above ₹1 lakh in the wrong table

Inter-state supplies to unregistered buyers above a threshold must be reported invoice-wise in Table 5, not rolled into the consolidated Table 7 figure. That threshold was ₹2.5 lakh for years. Notification 12/2024-Central Tax dated 10 July 2024 cut it to ₹1 lakh.

For a manufacturer selling direct to unregistered dealers or contractors in a neighbouring state, this quietly converted a lot of Table 7 lines into Table 5 lines. Accounting software configured before mid-2024 and never revisited is still using the old cut-off.

Cost: mis-declared place-of-supply data, which is exactly the kind of pattern that surfaces in a scrutiny notice. Fix: check the threshold your billing system uses for B2C-large classification. It is one configuration value, and it is wrong in a surprising number of installations.

Mistake 5 · Letting GSTR-1 liability run ahead of GSTR-3B

This is the one that generates an actual notice. Rule 88C compares the liability declared in GSTR-1 against the tax discharged in GSTR-3B for the same period. Where GSTR-1 is materially higher, the system issues an intimation in Form DRC-01B. You have seven days to either pay the difference with interest or explain it in Part B of the form.

Miss that window and the consequence is not a penalty — it is a block. Your GSTR-1 for the next period will not file. And because Rule 59(6) already blocks GSTR-1 when the previous period’s GSTR-3B is unfiled, one unattended intimation can freeze the entire outward chain, which freezes every customer’s credit behind it.

The usual innocent cause is double counting. E-invoices auto-populate into GSTR-1; someone also uploads the sales register manually; turnover in GSTR-1 is now higher than the tax actually paid.

Cost: seven-day clock, then GSTR-1 blocked — and your customers’ ITC blocked with it. Fix: reconcile the IRN list from the e-invoice portal against the sales register before upload. Every document should appear exactly once.

Three more that show up in scrutiny

The month-end sequence that prevents all five

None of this requires more effort than most factories already spend. It requires the effort to happen in a different order — reconciliation before GSTR-1 rather than between GSTR-1 and GSTR-3B:

  1. Tie the sales register to the IRN list. Pull the month’s IRNs from the e-invoice portal and match them against your sales register. Every document once, no more, no fewer. This alone prevents Mistake 5.
  2. Validate masters, not invoices. Customer GSTINs verified against the portal; every item carrying a valid HSN at the right digit length and a current GST rate. Mistakes 2 and 3 are both master-data failures wearing a return-filing costume.
  3. File GSTR-1 by the 11th (or the IFF by the 13th on QRMP), with B2C inter-state invoices above ₹1 lakh sitting invoice-wise in Table 5.
  4. Read the auto-drafted GSTR-3B before you file it. Compare tables 3.1 and 3.2 against your books. Anything wrong goes into GSTR-1A now — one shot, before GSTR-3B.
  5. File GSTR-3B by the 20th against a liability that already matches. Rule 88C finds no gap, and no DRC-01B is generated.

Step 4 is the one that feels new and is actually the whole change. The auto-drafted GSTR-3B used to be a suggestion. It is now a verdict, and the review has to happen while GSTR-1A is still open.

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Why manufacturers get caught more than traders

A trading business raises invoices from one register. A factory raises them from several places that were never designed to agree with each other: the despatch desk cuts the tax invoice, the stores issue a job-work challan that carries tax values but is not a supply, the e-way bill is generated by whoever is nearest the computer, and the credit note for a rejected consignment is raised weeks later after the quality dispute settles.

Each of those is a legitimate document. The problem is that GSTR-1 wants them as one consistent set, and in most SME factories they live in three systems and a WhatsApp group. When the outward return was a disclosure, the seams did not matter much. Now that it sets your liability and gates next month’s filing, every seam is a place where a notice can start.

This is the practical case for keeping despatch, invoicing, e-invoice and returns on one record rather than four — not tidiness, but the fact that the reconciliation in step 1 above becomes something the system already knows instead of a spreadsheet somebody builds on the 9th.

The bottom line

GSTR-1 used to be a form you filed. It is now the document that decides what you owe, whether you can file next month, and whether your customers get their credit. The five mistakes above are not exotic — four of them are data-entry issues and one is a configuration value — but the system that used to absorb them quietly has been removed.

If your month-end still involves reconciling GSTR-1 against GSTR-3B after both are filed, that is the process to change first. Everything else follows from moving that check to the front.

Related reading: Job Work and ITC-04 for the challan side of outward movement, How to Generate a GST E-Invoice in 2026 for the IRN process that feeds GSTR-1, and 5 Things Tally Cannot Do That a Manufacturing ERP Can if the seams described above sound familiar.

FAQ

Can I still fix a GSTR-1 mistake in GSTR-3B?

No. Since the July 2025 tax period the outward-liability tables of GSTR-3B (3.1 and 3.2) are auto-populated from GSTR-1 and GSTR-1A and are non-editable. The only correction route before you file GSTR-3B is GSTR-1A for the same period. After that, the fix moves to an amendment table in a later GSTR-1.

What is the deadline for filing GSTR-1A?

GSTR-1A opens after you file GSTR-1 or after the GSTR-1 due date, whichever is later, and closes when you file GSTR-3B for the same tax period. It can be filed only once per period, so it has to be right the first time.

Can GSTR-1A be used to correct a wrong customer GSTIN?

No. The recipient’s GSTIN is the one field GSTR-1A cannot amend. A wrong buyer GSTIN has to be corrected through the B2B amendment table of a later GSTR-1, which means your customer does not see the invoice in their GSTR-2B until that later period.

How many HSN digits does GSTR-1 Table 12 require?

Four digits if your previous-year aggregate turnover is up to ₹5 crore and six digits if it is above ₹5 crore. Since the Phase 3 rollout, HSN codes must be picked from the portal dropdown — manual typing is disabled — and Table 12 is split into separate B2B and B2C tabs that have to reconcile with the invoice tables.

What is DRC-01B and when is it issued?

DRC-01B is a system-generated intimation under Rule 88C of the CGST Rules, issued when the liability declared in GSTR-1 exceeds the liability discharged in GSTR-3B by more than the prescribed threshold. You get seven days to pay the difference or explain it in Part B. Ignore it and your GSTR-1 for the next period is blocked.

What is the late fee for filing GSTR-1 late?

₹50 per day for a return with data and ₹20 per day for a nil return. The maximum is ₹2,000 for aggregate turnover up to ₹1.5 crore, ₹5,000 between ₹1.5 crore and ₹5 crore, and ₹10,000 above ₹5 crore, with nil returns capped at ₹500. The real cost is rarely the late fee — it is the customer whose input tax credit is stuck.

Is there a time limit after which GSTR-1 can no longer be filed at all?

Yes. Under the Finance Act 2023 amendments to sections 37, 39, 44 and 52 of the CGST Act, a return cannot be filed once three years have passed from its due date. GSTN implemented the bar on the portal from the November 2025 return period onwards, and it applies to GSTR-1, GSTR-1A, GSTR-3B, GSTR-9 and others. Once a period is barred, the outward supplies in it never reach your customers’ GSTR-2B.

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Not tax advice

This article summarises GST procedure as it stands in August 2026 and is written for factory owners and accounts teams, not as a substitute for your CA. Rates, thresholds and portal behaviour change by notification — check the CBIC and GSTN sources below before acting on any specific filing.

Primary sources cited